tree 69fc54d4dd37f6a7018ba33e83a2989345850c1e
parent 22a5a1e7ddd6435234982c86615f437de79fccf3
author Martin Storsjö <martin@martin.st> 1652257238 +0300
committer Copybara-Service <copybara-worker@google.com> 1654547473 -0700

[libunwind] Don't store a predecremented PC when using SEH

This fixes unwinding in boundary cases on ARM with SEH.

In the case of ARM/Thumb, disp->ControlPc points at the following
instruction, with the thumb bit set. Thus by decrementing 1,
it still points at the next instruction. To achieve the desired
effect of pointing at the previous instruction, one first has to strip
out the thumb bit, then do the decrement by 1 to reach the previous
instruction.

When libcxxabi looks for call site ranges, it already does
`_Unwind_GetIP(context) - 1` (in `scan_eh_tab` in
libcxxabi/src/cxa_personality.cpp), so we shouldn't do the
corresponding `- 1` multiple times.

In the case of libcxxabi on Thumb, `funcStart` (still in `scan_eh_tab`)
may have the thumb bit set. If the program counter address is
decremented both in libunwind (first removing the thumb bit, then
decremented), and then libcxxabi decrements it further, and compares
with a `funcStart` with the thumb bit set, it could point to one byte
before the start of the call site.

Thus: This modification makes libunwind with SEH work with libcxxabi
on Thumb, in settings where libunwind and libcxxabi worked fine with
Dwarf before.

For existing cases with libunwind with SEH (on x86_64 and aarch64),
this modification doesn't break any of my testcases.

Differential Revision: https://reviews.llvm.org/D126869

GitOrigin-RevId: 08d30c602bcf3bc25a02938f2663a1b9e96a7cbe